The present invention relates to a method of making a hybrid optical master disc, and more particularly to a method of forming a watermark image in a hybrid optical master disc and replicating the watermark image in hybrid optical disks which are derived from the master disc.
Hybrid optical recording discs are discs having a read-only memory (ROM) area and a recordable area for recording or writing data which are usually generated by a computer user and which are recorded on the disc by a recorder controlled by a computer. Such a disc has a substrate which can be formed by injection molding against a master plate (also called a stamper) so that the substrate will have a continuous spiral groove extending from an inner edge to an outer edge of the substrate. The spiral groove is usually a groove which provides data channels on the disc and also provides for tracking of the disc while reading or recording data. The groove is frequency-modulated in a direction normal to the groove and is, therefore, referred to as a wobbled groove or a wobble groove. In the ROM area of a hybrid optical disc the groove is further modulated in the form of depressions which correspond to disc addressing data and to disc program data. The mastered substrate is then coated with a recording medium structure which can include an organic dye layer selected to absorb radiation from an optical recorder to further modify the depressions in the tracks of the ROM area, or to modulate the tracks in the recordable area. Upon coating the dye layer, a reflective layer is formed over this layer. A protective layer, usually of a polymer organic material, is formed over the reflective layer.
The groove of a hybrid optical recording disc, the degree of modulation of the groove, as well as the arrangement of addressing and program data is usually provided in accordance with Orange Book specifications. xe2x80x9cOrange Bookxe2x80x9d is a specification published by Philips Corporation and Sony Corporation which defines key properties of recordable compact disc media and recording performance.
The aforementioned master plate is used to produce numerous plastic disc substrates by embossing or injection molding techniques. Such a master plate is also referred to as a stamper. The stamper, in turn, is produced by metal plating techniques whereby a photoresist master disc having the spiral groove and the data depressions are plated with a metal. The metal layer is then separated from the photoresist master disc and constitutes the stamper which replicates the features of the photoresist master disc in an inverted orientation, i.e. a groove in the photoresist master will be a projection in the stamper. The photoresist master disc comprises a photoresist layer formed over a substrate which is usually a glass substrate. Accordingly, the photoresist master disc is also referred to in the art as a glass master.
Various proposals have been advanced for providing optical storage media such as, for example, conventional optical discs or hybrid optical discs, with indicia which would uniquely identify such discs and which would offer some protection against counterfeiting or copying of an optical storage medium.
For example, Li discloses in U.S. Pat. No. 5,549,953 optical recording media having optically-variable security properties which are achieved by introducing thin film structures having both optically-variable security properties and encoded optical data decodable by optical means. The thin film structures comprise a substrate and a multilayer interference coating carried by the substrate. The encoded data is in the form of bar codes or digital data.
Sullivan et al. describe in U.S. Pat. No. 5,510,163 optical storage media having a visible logo which is produced by a logo coating. The logo coating is sufficiently light absorbing in the visible spectrum to provide a visible contrast between a recording coating and the logo coating.
Thus, the above disclosures are directed to copy protection for optical media. Such protection is provided by introducing thin film structures or a logo coating into an optical medium. While introducing such structures or coatings into or onto an optical recording medium or an optical storage medium provides the intended security features, introduction of additional materials into or onto a recording layer is likely to increase the level of complexity of fabrication of such media and result in a corresponding increase of cost of such media.
It is an object of the present invention to provide an optical recording or storage medium having medium identification indicia which also offer protection against copying, with the indicia formed directly in an optical recording layer or in an optical storage layer of the medium.
It is an object of the present invention to provide a method of forming a watermark image in a photoresist layer of a hybrid optical master disc.
These and other objects and advantages of the present invention are achieved in a method of forming a watermark image in a hybrid optical master disc for replication in a hybrid optical disc, comprising the steps of:
a) providing a master substrate;
b) coating a photoresist layer over a surface of the master substrate;
c) exposing at a laser beam groove exposure level a continuous spiral wobbled groove in the photoresist layer, the groove extending from a designated read-only memory (ROM) area to a designated recordable area of the master disc, and exposing at a laser beam depression exposure level depressions in the groove in the ROM area in response to data to be recorded therein;
d) projecting a watermark image over at least portions of the ROM area and the recordable area at a watermark exposure level which is lower than the groove exposure level; and
e) developing the photoresist layer to provide the continuous spiral wobbled groove, the depressions in the ROM area, and portions of the watermark image remaining in lands between successive turns of the spiral groove.
These and other objects and advantages of the present invention are also achieved in a method of forming a watermark image in a hybrid optical master disc for replicating in a hybrid optical disc, comprising the steps of:
a) providing a master substrate;
b) coating a photoresist layer over a surface of the master substrate;
c) exposing at a laser beam groove exposure level a continuous spiral wobbled groove in the photoresist layer, the groove extending from a designated read-only memory (ROM) area to a designated recordable area of the master disc, and exposing at a laser beam depression exposure level depressions in the groove in the ROM area in response to data to be recorded therein;
d) projecting a watermark image over at least portions of the ROM area and the recordable area at a watermark exposure level which is lower than the groove exposure level;
e) developing the photoresist layer to provide the continuous spiral wobbled groove, the depressions in the ROM area, and portions of the watermark image remaining in lands between successive turns of the spiral groove;
f) making a metal stamper by plating a metal into the developed photoresist layer;
g) replicating the continuous spiral wobbled groove, the depressions in the groove in the ROM area, and the remaining watermark image into a plastic hybrid optical disc substrate by impressing the stamper upon a surface of the plastic substrate; and
h) coating the hybrid optical disc substrate with a recording medium structure to provide a hybrid optical disc having a watermark image extending over a portion thereof on the disc substrate under the recording medium structure.
It is a feature of the present invention to provide a method of making a hybrid optical disc with a watermark image which can be accomplished in the process of stamping a spiral groove and depressions in such groove.
It is another feature of the present invention to provide a method of exposing a photoresist layer of a hybrid optical master disc with a watermark image having a halftone portion of exposed and unexposed domains with a spacing between exposed domains being substantially smaller than a spacing between adjacent portions of an exposed spiral wobbled groove and the exposed domains of the watermark image being exposed at an intensity level of a light source which is substantially lower than an intensity level of a laser light beam which is used to exposed the spiral wobbled groove.
It is a still further feature of the present invention to provide a method of making hybrid optical discs by replicating a spiral wobbled groove, depressions in the groove in a ROM area, and a watermark image, developed in a photoresist layer of a hybrid optical master disc, wherein the replicated watermark image is visually discernible but not readable by a reading laser light beam, the watermark image thereby identifying a hybrid optical disc and the watermark image being substantially non-copyable.
It is another feature of the present invention to provide a method of exposing a watermark image in a photoresist layer of a hybrid optical master disc before or after exposing the photoresist layer with a spiral wobbled groove in a ROM area and in a recordable area of the master disc and exposing depressions in the exposed groove in the ROM area of the master disc.